Investigate and prototype interfaces to externalize space usage and related metrics

This is an umbrella feature request to productize and externalize existing internal methods that track space usage and related metrics. It's likely that we may need to add a few more metrics, but here is an initial straw man list.

1. Total space of the device
2. Total allocated space; i.e. space consumed by all allocated extents
3. Total used / unused space; i.e. space actually used / still free by pages in allocated extents
4. Total free space; i.e. space left-over in unallocated extents
5. Number of extents allocated
6. Data v/s log space consumption; allocated, used / unused etc.
7. Quick summary of row-counts: # of rows inserted

Some of these ideas have been aired out in this internal slack post. Feedback suggests that these are useful and do-able.

Investigate and prototype how-to extend upcoming splinterdb-cli to provide interfaces to report such metrics.